Modern broadcast communications systems transmit and receive information by way of transmitters that modulate a radio frequency (RF) carrier signal with an information signal. The information signal can be at a much lower frequency than the RF signal. Corresponding receivers then demodulate the transmitted signal to recover the information signal.
Such RF receivers typically use some form of heterodyning to convert a received RF signal to a lower frequency signal (sometimes called an intermediate frequency signal), which may be easier to filter. Generally, heterodyning refers to a process of mixing (or multiplying) a first signal with a second signal having a frequency that is close to that of the first signal. In this instance, the RF signal is multiplied with a local oscillator signal (LO signal). Mixing the two signals results in two signals, a first signal having a frequency equal to the sum of the RF frequency and the LO frequency, and a second signal having a frequency equal to the difference between the RF and the LO frequencies. The first frequency is higher than either the RF or LO frequency, and is usually filtered readily using a simple low-pass filter. The difference frequency is the intermediate frequency (IF), which can be manipulated using fixed frequency filters.
Unfortunately, typical heterodyne-based systems are susceptible to a phenomenon referred to as imaging. Imaging refers to a signaling phenomenon whereby two different RF signals are translated to the same intermediate frequency, thereby causing interference. In general, a desired RF frequency fRF differs from a given LO signal frequency fLO by the IF frequency fIF. A desired radio frequency may lie either above or below the LO signal frequency. However, due to its symmetric properties, heterodyning systems sometimes select any RF signal differing from fLO by fIF, regardless of whether the RF signal lies above or below fLO. For example, if a desired RF signal has a frequency of 1.01 GHz and the LO signal has a frequency of 1.00 GHz, the two signals can be mixed to produce an IF signal having an IF frequency of 10 MHz. However, if there is a second RF signal with a frequency of about 990 MHz, the receiver will mix both the 1.01 GHz and the 990 MHz signals to the same frequency of 10 MHz, thereby causing interference with the desired signal. The image frequency can be, for example, the frequency corresponding to the sum of fLO and fIF.
To prevent interference with the desired RF signal, some communication systems use quadrature receiver architectures for mixing the RF signal using two local oscillator signals in quadrature with each other. One of the paths is typically referred to as an in-phase (I) signal path, and the other path is typically referred to as a quadrature (Q) signal path.
If the LO signal applied to the I signal path is exactly 90 degrees out of phase with the LO signal applied to the Q signal path, and if the I path and the Q path circuits are identically matched in terms of amplitude and phase, then the image signal can be perfectly rejected from the desired signal. This property allows quadrature IF mixing to cancel image signals without expensive and bulky rejection filters. However, if any non-idealities exist in the LO signals (e.g. imperfect 90 degree phase difference) or if the I and Q paths are imbalanced or mismatched (phase, amplitude, and so on), then the gain and phase of the I/Q path circuits will cause the image signal to leak into the desired signal, resulting in imperfect image cancellation.
To improve image rejection or cancellation, some receivers are calibrated by using a calibration tone to measure mismatches in the I and Q paths. Then the I path or the Q path is compensated in response to the measured mismatch, improving image rejection.
Recently, advances in integrated circuit technology have allowed nearly all of a complete receiver to be integrated onto a single silicon chip. One of the problems introduced by such high degree of integration has been that the LO mixing signals produced by oscillators can radiate into adjacent circuitry, creating spurs or tones that degrade performance. Richard A. Johnson in U.S. Pat. No. 6,778,117 discloses a receiver that uses a direct digital frequency synthesizer rather than a conventional oscillator to form a digital mixing signal. Since there is no circuit node that contains an actual oscillator signal, there is no mechanism for the local oscillator signal to leak or radiate into other circuits.
It would be desirable to utilize this and other advances in receiver design to provide a receiver with high image rejection and low cost.